In recent years, in a sequentially moving type exposure apparatus such as a stepper or a scanning stepper (also called a scanner), a spectrum narrowed pulse oscillation excimer laser of a discharged pumped type is used as the light source. The pulse beam oscillated from the excimer laser passes the illumination optical system of the exposure apparatus main body and is irradiated on the photosensitive agent coated on the surface of the wafer mounted on the wafer stage exposing the photosensitive agent, and the wafer is exposed.
As for the spectral width of a laser beam, narrowing is required from the point of optical system design of the exposure apparatus, especially from the point of permissible chromatic aberration. Regarding the quality level of such narrowing, conventionally, a specific upper limit value was usually set to the spectral width, and as for the laser used in the light source of an exposure apparatus, maintaining the spectral width under the above specific upper limit while including changes due to various causes was required. Narrowing to a desired spectral width is achieved by using optical elements such as a grating, an etalon (a Fabry-Perot etalon).
Further, conventionally, the level of sensitivity of the exposure apparatus image-forming performance with respect to the variation amount of the spectral width of the laser beam could be ignored.
The present situation surrounding the sequentially moving type exposure apparatus is that due to finer patterns subject to image-forming corresponding to higher integration of semiconductor devices or the like, numerical aperture of the projection optical system (projection lens) is increasing (higher N.A.), and as a consequence, narrowing of the laser spectral is progressing to the ultimate level. Further, nowadays, according to finer device rules that accompany the finer patterns subject to image-forming (subject to resolution), the sensitivity of the image-forming with respect to the spectral change has reached a level that can no longer be ignored.
Especially the influence that the spectral width change of the laser beam has on the line width of an isolated line is at a level that can no longer be ignored.
Further, in recent years, a mask correction (a reticle pattern correction) called optical proximity effect correction (OPC; Optical Proximity Correction) in which a desired pattern is obtained on a wafer by predicting the optical proximity effect (OPE) in advance and correcting the reticle pattern is actively performed. As the cause of linewidth error after the OPC, various causes that change spectral characteristics of the laser beam oscillated from the laser light source are also becoming a cause that cannot be ignored. As the cause of the change in spectral characteristics of the laser beam, manufacturing error of the narrowing element, energy state of a gain generator, emission angle characteristics and oscillation conditions (duty) of the resonator and the like can be representatively given.
Accordingly, with the laser unit used as a light source in an exposure apparatus, management and control of the spectral width of the laser beam are required more than ever, and at the same time, as for monitoring the spectral, the measurement accuracy has to be improved than before, and calibration errors of the monitoring have to be kept to a minimum.
Further, when manufacturing a semiconductor device, a plurality of exposure apparatus are used, therefore, it is desirable that errors are minimal in spectral characteristics of the laser beam oscillated from the light source among the exposure apparatus in the same manufacturing line.